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Chapter 2 Electrical Principles and PLCs

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1 Chapter 2 Electrical Principles and PLCs
Programmable Logic Controllers and Electrical Principles • Voltage • Current • Resistance • Series Circuits • Parallel Circuits

2 Basic rules must be followed when installing output devices to PLCs
Basic rules must be followed when installing output devices to PLCs. Output devices cannot be installed in series from PLC output terminals. PLC circuits and all equipment connected to and controlled by PLCs demonstrate electrical principles. PLC hardwired circuits and programs also make use of electrical principles as applied to series, parallel, and series/parallel connected components and circuits. See Figure 2-1. Understanding electrical principles is especially important when there is a problem in an electrical control system that requires troubleshooting, such as having output components wired in series. (Output components cannot be connected in series to PLC output modules.)

3 When all PLC system hardware is functioning but the system is not operating correctly, the problem may be with the software, such as an incorrect program, incorrect symbols, or the program not being keyed correctly into the PLC. PLC problems typically fall into one of two categories: software and hardware. A PLC software problem is a condition where a PLC is properly installed and all inputs and outputs are working, but there is a problem with the PLC program. The PLC cannot execute the programmed instructions. See Figure 2-2.

4 A PLC hardware problem is caused by problems with input devices, output devices, wiring, or a short or open somewhere in the system. A PLC hardware problem is a condition where a PLC program is correct for the application, but the system is not working properly because there is a problem with inputs, outputs, wiring (loose connections, cross talking, incorrect wiring), a short or open circuit exists somewhere in the system, or another hardware problem exists. See Figure 2-3. Understanding electrical principles is important when testing an electrical system or individual components and deciding what corrective action to take if a problem is found. For any troubleshooting to be successful, knowing what voltage, current, power, and resistance values should be when taking a measurement is essential.

5 The voltage supplied to portable PLCs can come from portable generators, batteries, or solar panels.
Voltage is produced by electromagnetism (generators), chemicals (batteries), light (photocells), heat (thermocouples), pressure (piezoelectricity), or friction (static electricity). Most PLC electrical systems operate from standard power delivered by a local utility company, but some manufacturers create electrical power using generators. PLCs that control remote or portable equipment can be powered by batteries or a combination of batteries and solar power, such as with portable traffic lights used in a construction zone. See Figure 2-4.

6 All points in a DC voltage circuit have polarity.
All DC voltage sources have a positive and negative terminal. The positive and negative terminals establish polarity in a circuit. Polarity is the positive (+) or negative (–) state of an object. DC positive polarity is also called the source, or supply. DC negative polarity is also called the common, or sink. See Figure 2-5. All points in a DC circuit have polarity. Understanding the positive and negative points of a DC circuit is important when wiring DC devices and components to the input and output terminals of a PLC.

7 AC voltages are produced by generators, which create single-phase or three-phase sine waves when rotated. AC voltage is the most common voltage used to produce work. AC voltage is produced by generators that create an AC sine wave when rotated. An AC sine wave is a symmetrical waveform that contains 360 electrical degrees and has one positive alternation and one negative alternation per cycle. A cycle is one complete positive alternation and one complete negative alternation of a waveform. An alternation is half of a cycle. A sine wave reaches a peak positive value at 90°, returns to 0 V at 180°, increases to a peak negative value at 270°, and returns to 0 V at 360°. See Figure 2-6.

8 PLCs are powered by single-phase AC but can control both single-phase or three-phase AC loads.
Single-phase power lines are marked L1 and L2 or N. Three-phase power lines are marked L1 (A or R), L2 (B or S), and L3 (C or T). Single-phase power is used for low-voltage loads and three-phase power is used for high-voltage loads. PLCs use single-phase voltage as a power supply and to control any AC-rated inputs or outputs. See Figure 2-7. However, single-phase power from the output of a PLC can also be used to control a three-phase motor starter. Likewise, single-phase power from a PLC can be used to control a relay or heating contactor that controls a three-phase heater.

9 Low, medium, and high voltages are used in commercial and industrial PLC applications.
Low AC voltages (6 V to 24 V) are used for doorbells, security systems, and as a voltage level for PLC inputs. Medium AC voltages (115 V) are used in residential applications for lighting, heating, cooling, cooking, running motors, as the power supply voltage to security system PLCs, and as the voltage level of some home monitoring PLC inputs and outputs. See Figure 2-8. Using 115 VAC for PLC input and output terminals is common when a PLC is used to control a circuit that includes only 115 VAC loads.

10 PLCs have power supply voltage ratings that can be a fixed voltage rating, a voltage range, or a dual voltage rating. All PLCs must receive power to operate. Power supplied to a PLC is used to operate its internal circuitry (CPU) and possibly supply voltage to the output and input terminals of the PLC. PLCs have a supply voltage rating that can be a fixed rating, such as 115 VAC, or a voltage range, such as 85 VAC to 265 VAC. Some PLCs have a dual voltage rating of 85 VAC to 132 VAC or 170 VAC to 265 VAC that is set by the placement of a jumper. See Figure 2-9. Even when a PLC has a fixed voltage rating, the voltage applied to the PLC can still vary within the standard +5% to –10% that applies to most fixed-voltage-rated electrical equipment.

11 PLC input terminals are available with DC, AC, or DC/AC ratings that typically use low voltages such as 12 VDC or 24 VDC. Low voltage rated input terminals (12 VDC or 24 VDC) are the most common and preferred type of PLC input terminals. Lower voltages are safer and provide for an easier installation because lower voltages do not require input wires to be run in conduit. Many PLCs provide a 12 VDC or 24 VDC output power terminal that can be used to power input terminals. See Figure 2-10.

12 The output terminal switches of a PLC are either mechanical switches or solid-state switches.
The output terminal switches of a PLC are either mechanical contact switches or solid-state switches. See Figure The advantage of a mechanical contact output terminal is that a mechanical contact can switch either AC or DC loads. An AC solid-state output terminal of a PLC can only switch AC loads and a DC solid-state output terminal can only switch DC loads. Solid-state output switches of PLCs include triacs for switching AC circuits and transistors and SCRs for switching DC circuits.

13 Typical PLC input circuitry draws about 5 mA to 20 mA of current.
The amount of current in an input circuit of a PLC is low because the internal circuitry of a PLC input section is the load of the input circuit. The internal circuitry only requires a few mA of electron flow to activate. A typical PLC input section only draws about 5 mA to 20 mA of current. However, low-current inputs of a PLC are capable of controlling high-current outputs. See Figure 2-12.

14 A two-wire input switch circuit uses mechanical (pushbutton) or solid-state (photoelectric and proximity) switches, and a three-wire input switch circuit uses solid-state switches. The two types of DC input switches are two-wire and three-wire. See Figure A two-wire input switch circuit uses mechanical switches (limit) or solid-state (photoelectric and proximity) switches. A three-wire input switch circuit uses solid-state switches.

15 Switching devices that use NPN transistors as switching elements are called current sinking, negative switching, or NPN devices. When using an NPN transistor-type input device, the load (PLC input section) is connected between the positive terminal of the supply voltage and the output terminal (collector) of the switch or sensor. When a switch such as a photoelectric or proximity sensor detects a target, electron flows through the transistor switch, energizing the input terminal. See Figure 2-14.

16 Switching devices that use PNP transistors as the switching elements are called current sourcing, positive switching, or PNP devices. When using a PNP transistor-type input device, the load (PLC input section) is connected between the negative terminal of the supply voltage and the output terminal (collector) of the switch or sensor. When the switch detects a target, electron flows through the transistor and the PLC input terminal is energized. See Figure 2-15.

17 The output sections of PLCs use transistors, triacs, and mechanical or solid-state relays for switching. PLC output components connected to the output section of a PLC determine the amount of current a PLC must carry. The amount of current drawn by an output component can vary from a few mA (small indicating lamps), to several amps (solenoids, higher wattage lamps, small motors, and large-motor motor starters), to hundreds of amps (large three-phase motors and three-phase heating elements). Large current output components must be connected to a PLC through an interface device. See Figure 2-16.

18 Copper (Cu) is a better conductor than aluminum (Al) because copper can carry more current for a given size (AWG). A conductor with a large cross-sectional area has less resistance than a conductor with a small cross-sectional area. See Figure A large conductor can also carry more current. The longer a conductor is, the greater the resistance of the conductor. Short conductors have less resistance than long conductors of the same size (AWG). Copper (Cu) is a better conductor than aluminum (Al) and can carry more current for a given size (AWG). Temperature also affects resistance. For metals, the higher the temperature, the greater the resistance. The higher the operating temperature of a wire, the lower the ampacity rating of the wire.

19 Series circuits have two or more devices or components connected so there is only one flow path for current to take. Fuses, switches, loads, conductors, and other electrical devices and components can be connected in series. Series connected devices or components are two or more devices or components that are connected so that there is only one flow path for current to take. Opening a series circuit at any point stops the flow of current. Current stops flowing any time a fuse blows, a circuit breaker trips, or a switch or load opens. See Figure 2-18.

20 PLC communications cables connected in series (daisy-chained) are used to connect individual PLC processors. PLC circuits include series-connected components. All fuses of a PLC are connected in series with the power supply. The power supply feeds power to the input and output sections, with all electrons flowing from the power supply through the fuses. Likewise, some PLC communications cables used to connect separate processors together are connected in series (also called daisy-chained). See Figure 2-19.

21 The total resistance of a series circuit increases when loads are added in series and decreases when loads are removed. The total resistance in a circuit containing series-connected loads equals the sum of the resistances of all the loads. The total resistance in a series circuit increases when loads are added in series and decreases when loads are removed. Loose connections and switching contacts also have resistance, which adds to the total circuit resistance. See Figure 2-20.

22 Electron flow in a series circuit is the same everywhere in the circuit.
The current in a circuit containing series-connected loads is the same throughout the circuit. The same amount of current that the load draws is the same amount of current through the switch controlling the load and the fuse protecting the circuit. See Figure 2-21.

23 Voltage drop is the amount of voltage consumed by a device or component as current passes through it. The total voltage applied across loads connected in series is divided across the individual loads that have resistance, no matter how low the resistance of the component. Each load drops a set percentage of the applied voltage. Voltage drop is the amount of voltage consumed by a device or component as current passes through the object. The exact voltage drop across each load depends on the resistance of each specific load. The voltage drops across any two loads are the same if the resistance values of the loads are the same. See Figure 2-22.

24 Parallel circuits have two or more devices or components connected so that there is more than one flow path for current to take. Fuses, switches, loads, conductors, and other devices and components can be connected in parallel. Parallel-connected devices and components are two or more devices or components that are connected so that there is more than one flow path for current to take. Input devices (switches) and output components (loads) can be connected in parallel. See Figure 2-23.

25 Total resistance of a parallel circuit decreases when loads are added in parallel and increases when loads are removed. The total resistance in a circuit containing parallel-connected loads is less than the lowest resistance value of any given load. The total resistance decreases if loads are added in parallel and increases if loads are removed. See Figure 2-24.

26 Total current in a parallel circuit equals the sum of the current through all loads in the parallel circuit or leg. Total current in a circuit containing parallel-connected loads equals the sum of the current through all the loads. Total current increases when loads are added in parallel and decreases when loads are removed. See Figure 2-25.

27 The voltage drop across individual loads remains the same when parallel loads are added or removed.
The voltage drop across each load is the same when loads are connected in parallel. The voltage drop across each load remains the same when parallel loads are added or removed. See Figure 2-26.


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